.TH std::unique_ptr 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::unique_ptr \- std::unique_ptr

.SH Synopsis
   Defined in header <memory>
   template<

       class T,                               \fB(1)\fP \fI(since C++11)\fP
       class Deleter = std::default_delete<T>

   > class unique_ptr;
   template <

       class T,                               \fB(2)\fP \fI(since C++11)\fP
       class Deleter

   > class unique_ptr<T[], Deleter>;

   std::unique_ptr is a smart pointer that owns and manages another object through a
   pointer and disposes of that object when the unique_ptr goes out of scope.

   The object is disposed of, using the associated deleter when either of the following
   happens:

     * the managing unique_ptr object is destroyed.
     * the managing unique_ptr object is assigned another pointer via operator= or
       reset().

   The object is disposed of, using a potentially user-supplied deleter by calling
   get_deleter()(ptr). The default deleter uses the delete operator, which destroys the
   object and deallocates the memory.

   A unique_ptr may alternatively own no object, in which case it is called empty.

   There are two versions of std::unique_ptr:

    1. Manages a single object (e.g. allocated with new).
    2. Manages a dynamically-allocated array of objects (e.g. allocated with new[]).

   The class satisfies the requirements of MoveConstructible and MoveAssignable, but of
   neither CopyConstructible nor CopyAssignable.

.SH Type requirements
   -
   Deleter must be FunctionObject or lvalue reference to a FunctionObject or lvalue
   reference to function, callable with an argument of type unique_ptr<T,
   Deleter>::pointer.

.SH Notes

   Only non-const unique_ptr can transfer the ownership of the managed object to
   another unique_ptr. If an object's lifetime is managed by a const std::unique_ptr,
   it is limited to the scope in which the pointer was created.

   std::unique_ptr is commonly used to manage the lifetime of objects, including:

     * providing exception safety to classes and functions that handle objects with
       dynamic lifetime, by guaranteeing deletion on both normal exit and exit through
       exception.
     * passing ownership of uniquely-owned objects with dynamic lifetime into
       functions.
     * acquiring ownership of uniquely-owned objects with dynamic lifetime from
       functions.
     * as the element type in move-aware containers, such as std::vector, which hold
       pointers to dynamically-allocated objects (e.g. if polymorphic behavior is
       desired).

   std::unique_ptr may be constructed for an incomplete type T, such as to facilitate
   the use as a handle in the pImpl idiom. If the default deleter is used, T must be
   complete at the point in code where the deleter is invoked, which happens in the
   destructor, move assignment operator, and reset member function of std::unique_ptr.
   (Conversely, std::shared_ptr can't be constructed from a raw pointer to incomplete
   type, but can be destroyed where T is incomplete). Note that if T is a class
   template specialization, use of unique_ptr as an operand, e.g. !p requires T's
   parameters to be complete due to ADL.

   If T is a derived class of some base B, then std::unique_ptr<T> is implicitly
   convertible to std::unique_ptr<B>. The default deleter of the resulting
   std::unique_ptr<B> will use operator delete for B, leading to undefined behavior
   unless the destructor of B is virtual. Note that std::shared_ptr behaves
   differently: std::shared_ptr<B> will use the operator delete for the type T and the
   owned object will be deleted correctly even if the destructor of B is not virtual.

   Unlike std::shared_ptr, std::unique_ptr may manage an object through any custom
   handle type that satisfies NullablePointer. This allows, for example, managing
   objects located in shared memory, by supplying a Deleter that defines typedef
   boost::offset_ptr pointer; or another fancy pointer.

       Feature-test macro      Value    Std            Feature
   __cpp_lib_constexpr_memory 202202L (C++23) constexpr std::unique_ptr

.SH Member types

   Member type  Definition
   pointer      std::remove_reference<Deleter>::type::pointer if that type exists,
                otherwise T*. Must satisfy NullablePointer
   element_type T, the type of the object managed by this unique_ptr
   deleter_type Deleter, the function object or lvalue reference to function or to
                function object, to be called from the destructor

.SH Member functions

   constructor   constructs a new unique_ptr
                 \fI(public member function)\fP
   destructor    destructs the managed object if such is present
                 \fI(public member function)\fP
   operator=     assigns the unique_ptr
                 \fI(public member function)\fP
.SH Modifiers
   release       returns a pointer to the managed object and releases the ownership
                 \fI(public member function)\fP
   reset         replaces the managed object
                 \fI(public member function)\fP
   swap          swaps the managed objects
                 \fI(public member function)\fP
.SH Observers
   get           returns a pointer to the managed object
                 \fI(public member function)\fP
   get_deleter   returns the deleter that is used for destruction of the managed object
                 \fI(public member function)\fP
   operator bool checks if there is an associated managed object
                 \fI(public member function)\fP
.SH Single-object version, unique_ptr<T>
   operator*     dereferences pointer to the managed object
   operator->    \fI(public member function)\fP
.SH Array version, unique_ptr<T[]>
   operator[]    provides indexed access to the managed array
                 \fI(public member function)\fP

.SH Non-member functions

   make_unique
   make_unique_for_overwrite   creates a unique pointer that manages a new object
   \fI(C++14)\fP                     \fI(function template)\fP
   (C++20)
   operator==
   operator!=
   operator<
   operator<=                  compares to another unique_ptr or with nullptr
   operator>                   \fI(function template)\fP
   operator>=
   operator<=>
   (removed in C++20)
   (C++20)
   operator<<(std::unique_ptr) outputs the value of the managed pointer to an output
   (C++20)                     stream
                               \fI(function template)\fP
   std::swap(std::unique_ptr)  specializes the std::swap algorithm
   \fI(C++11)\fP                     \fI(function template)\fP

.SH Helper classes

   std::hash<std::unique_ptr> hash support for std::unique_ptr
   \fI(C++11)\fP                    \fI(class template specialization)\fP

.SH Example


// Run this code

 #include <cassert>
 #include <cstdio>
 #include <fstream>
 #include <iostream>
 #include <locale>
 #include <memory>
 #include <stdexcept>

 // helper class for runtime polymorphism demo below
 struct B
 {
     virtual ~B() = default;

     virtual void bar() { std::cout << "B::bar\\n"; }
 };

 struct D : B
 {
     D() { std::cout << "D::D\\n"; }
     ~D() { std::cout << "D::~D\\n"; }

     void bar() override { std::cout << "D::bar\\n"; }
 };

 // a function consuming a unique_ptr can take it by value or by rvalue reference
 std::unique_ptr<D> pass_through(std::unique_ptr<D> p)
 {
     p->bar();
     return p;
 }

 // helper function for the custom deleter demo below
 void close_file(std::FILE* fp)
 {
     std::fclose(fp);
 }

 // unique_ptr-based linked list demo
 struct List
 {
     struct Node
     {
         int data;
         std::unique_ptr<Node> next;
     };

     std::unique_ptr<Node> head;

     ~List()
     {
         // destroy list nodes sequentially in a loop, the default destructor
         // would have invoked its `next`'s destructor recursively, which would
         // cause stack overflow for sufficiently large lists.
         while (head)
         {
             auto next = std::move(head->next);
             head = std::move(next);
         }
     }

     void push(int data)
     {
         head = std::unique_ptr<Node>(new Node{data, std::move(head)});
     }
 };

 int main()
 {
     std::cout << "1) Unique ownership semantics demo\\n";
     {
         // Create a (uniquely owned) resource
         std::unique_ptr<D> p = std::make_unique<D>();

         // Transfer ownership to `pass_through`,
         // which in turn transfers ownership back through the return value
         std::unique_ptr<D> q = pass_through(std::move(p));

         // p is now in a moved-from 'empty' state, equal to nullptr
         assert(!p);
     }

     std::cout << "\\n" "2) Runtime polymorphism demo\\n";
     {
         // Create a derived resource and point to it via base type
         std::unique_ptr<B> p = std::make_unique<D>();

         // Dynamic dispatch works as expected
         p->bar();
     }

     std::cout << "\\n" "3) Custom deleter demo\\n";
     std::ofstream("demo.txt") << 'x'; // prepare the file to read
     {
         using unique_file_t = std::unique_ptr<std::FILE, decltype(&close_file)>;
         unique_file_t fp(std::fopen("demo.txt", "r"), &close_file);
         if (fp)
             std::cout << char(std::fgetc(fp.get())) << '\\n';
     } // `close_file()` called here (if `fp` is not null)

     std::cout << "\\n" "4) Custom lambda-expression deleter and exception safety demo\\n";
     try
     {
         std::unique_ptr<D, void(*)(D*)> p(new D, [](D* ptr)
         {
             std::cout << "destroying from a custom deleter...\\n";
             delete ptr;
         });

         throw std::runtime_error(""); // `p` would leak here if it were a plain pointer
     }
     catch (const std::exception&)
     {
         std::cout << "Caught exception\\n";
     }

     std::cout << "\\n" "5) Array form of unique_ptr demo\\n";
     {
         std::unique_ptr<D[]> p(new D[3]);
     } // `D::~D()` is called 3 times

     std::cout << "\\n" "6) Linked list demo\\n";
     {
         List wall;
         const int enough{1'000'000};
         for (int beer = 0; beer != enough; ++beer)
             wall.push(beer);

         std::cout.imbue(std::locale("en_US.UTF-8"));
         std::cout << enough << " bottles of beer on the wall...\\n";
     } // destroys all the beers
 }

.SH Possible output:

 1) Unique ownership semantics demo
 D::D
 D::bar
 D::~D

 2) Runtime polymorphism demo
 D::D
 D::bar
 D::~D

 3) Custom deleter demo
 x

 4) Custom lambda-expression deleter and exception safety demo
 D::D
 destroying from a custom deleter...
 D::~D
 Caught exception

 5) Array form of unique_ptr demo
 D::D
 D::D
 D::D
 D::~D
 D::~D
 D::~D

 6) Linked list demo
 1,000,000 bottles of beer on the wall...

.SH See also

   shared_ptr smart pointer with shared object ownership semantics
   \fI(C++11)\fP    \fI(class template)\fP
   weak_ptr   weak reference to an object managed by std::shared_ptr
   \fI(C++11)\fP    \fI(class template)\fP
